1. Field of the Invention
The invention relates to frequency division networks having low phase noise and more particularly to frequency division networks in which a sinusoidal wave is converted to a digital format for digital frequency division, and then converted back to the sinusoidal format, the phase noise remaining low in the sinusoidal output.
2. Prior Art
The synthesis of plural frequencies in a radar or communications system may take several well known forms. In general, economy and performance dictate the selection. The carrier frequency of a modulated sinusoidal signal may be converted to another frequency with a mixer. This process has the disadvantage of requiring an additional local oscillator that must have better stability and lower spurious signals than those specified for the original carrier frequency if carrier quality is to be maintained. The cost and the number of components needed when several of these local oscillator signals are required for a triple conversion receiver or signal processor, are generally reduced when one sinusoidal signal is generated by a master crystal oscillator and this frequency is then multiplied and divided to provide suitable local oscillator signals. The carrier quality may also be maintained if the multipliers and dividers are properly designed.
Good phase noise performance is that aspect of carrier quality which is of primary concern in the design of the multipliers and dividers in a radar exciter for moving target radar systems (MTI). In an MTI radar system, low phase noise is required to distinguish stationary targets from those that are moving at a low velocity.
The radar exciter in MTI radar systems is preferably a "direct coherent synthesizer" for minimum phase noise. A "synthesizer" is a device that generates a large number of easily selected, accurately controlled stable frequencies. A "non-coherent" synthesizer consists of several independent oscillators each oscillator having a separate frequency reference. A "coherent" synthesizer generates plural frequencies directly by harmonic and subharmonic generators driven from a single crystal oscillator. "Indirect" coherent synthesizers, are those which also use a single crystal oscillator, but which have one or more phase-lock loops to maintain coherence. These are often used in communication systems to tune the system to any one of a hundred or more channels. Indirect coherent synthesizers do not inherently have poor phase noise, but they have the disadvantage of being slow in frequency hopping.
In MTI radar systems employing direct coherent synthesis, the stable frequencies are generated by an exciter consisting of a single master crystal oscillator driving frequency multipliers and dividers whose outputs are added or subtracted in mixers to provide the desired plurality of output frequencies. These systems avoid phase-lock loops of the indirect coherent synthesizers because of the accompanying high levels of phase noise.
In the exciter of an MTI radar system, the object is to add minimum additional phase noise in the multipliers and dividers to that of the master crystal oscillator. The stability of a master crystal oscillator is extremely good and the phase noise very low compared to all the other elements in the synthesizer. Accordingly, for optimum phase noise performance, the exciter relies to a maximum extent upon the inherent stability of the crystal oscillator. Secondarily, the phase noise performance of the necessary multipliers and dividers is optimized. With crystal oscillators, good phase noise performance is available up to approximately 100 mHz. However the radar transmitter requires much higher frequencies, so frequency multipliers must be included in the chain if only in minimum numbers. Theoretically the multipliers will degrade the phase noise by 20 Log n where n is the multiplication ratio. In addition to the increase in theoretical phase noise--which is an artifact of the definition--there will also be an unavoidable but minimizable phase noise contribution from each stage in the multiplier. The latter phase noise contribution is not an artifact and is due primarily to imperfection in the active device function and to a lesser extent due to thermal noise in the resistors and conductors. The former theoretical phase noise contribution, which is added to the phase noise of the crystal oscillator in multiplication, disappears when the frequency is subsequently reduced to the final IF frequency for synchronous detection. The detection process is thus not degraded by the temporary presence of the theoretical increase in phase noise and its eventual disappearance. One might add that when frequency division occurs the same artifact appears to improve divider performance, but the detection process is neither improved nor degraded by its temporary presence.
The phase noise contributions of the multipliers and dividers, due primarily to the active device noise --such as uncertainty in the onset of conduction in a transistor--do degrade the detection process and must be kept to a minimum if MTI performance is to be optimized.
In an exciter optimized for low phase noise, the master oscillator is operated at a relatively high frequency to reduce the number of multiplying stages required to achieve the highest frequencies of the exciter as stated earlier. Accordingly, the master crystal oscillator is selected for low phase noise at the highest possible frequency, typically up to 100 mHz (and in the present embodiments 60 mHz). Master crystal oscillators of recent design have phase noise properties, which are as good at these frequencies as those of crystal oscillators operating at 5 or 10 mHz of older designs.
The 50-100 mHz oscillator frequency selected to simplify the phase noise problem in multiplying to the higher frequencies required of the exciter, however, is much higher than is desirable for creating the density of signals desirable for an agile radar system. Accordingly, frequency division must take place, and frequency dividers are required which have minimum active device noise.
In known indirect coherent synthesizers using voltage controlled oscillators and programmable dividers, the phase noise limits of about -140 to -150 dBc/Hz at a 10 MHz offset referenced to the master oscillator frequency have been attained. A more desirable limit and one that is attainable by multipliers and dividers using direct coherent synthesis is 10 to 20 dB/Hz lower.
The present invention is directed to a frequency division network having minimum active device noise to provide improved phase noise performance in the exciter of an MTI radar system.